With MCVD technology, a core matrix of doped silica is built up layer by layer on the inner surface of a supporting tube of silica glass, the tube being spacedly surrounded by an axially movable annular heater or ring furnace as described, for example, in commonly owned U.S. Pat. No. 4,389,230. The furnace, which may be operating at a reaction temperature of at least 500.degree. C., is slowly reciprocated along the tube which is being rotated about its axis while a flow of reactants is passed therethrough to form the core matrix; the latter may have a thickness between 10 and 50.mu.. The process is particularly suitable for glasses of high silica content and enables the production of fibers with very low attenuation of the luminous energy transmitted therethrough. For single-mode fibers the core can be made of silica SiO.sub.2 admixed with germanium dioxide GeO.sub.2 as a dopant, with or without the further addition of phosphorus pentoxide P.sub.2 O.sub.5. The attenuation coefficient of a fiber of this character varies between about 0.2 and 0.15 db/Km within a wavelength range of 1.5 to 1.6.mu., referred to in the art as the third transmission window.
Once the deposition of the core matrix is completed, the resulting structure is collapsed on its axis by heating to a predetermined higher temperature. The collapse generally takes place from one end of the tube to the other, with progressive displacement of the furnace--now operating at this high temperature--to heat successive zones of the tube. The collapsing end of the core matrix assumes a concave shape to form a meniscus advancing codirectionally with the furnace until the core is completely solidified. The resulting preform is a transparent cylindrical rod with a diameter between about 10 and 30 mm; the fiber subsequently drawn therefrom, in which the supporting tube forms the cladding or sheath, may be about 125.mu. in diameter.
During the deposition of the core matrix as well as during the collapsing step, the dopant or dopants of the core material undergo both diffusion and evaporation, the latter particularly from the innermost layer or layers. Since the volatility of the dopants increases with temperature, such evaporation is especially intense just before the collapse and results in a paraxial dip of the refractive-index profile which is typical of fibers produced by the MCVD process. This dip in the vicinity of the axis restricts the bandwidth of multimode fibers but is even more disadvantageous with single-mode fibers whose core diameter is smaller for the transmission of a given wavelength and whose preform, therefore, has a higher ratio of inner to outer core diameter prior to collapse. The dip impairs the propagation and guidance properties of the fiber and increases losses due to a phenomenon termed microbending, compared with those of similar fibers produced by other techniques such as those known as outside vapor deposition (OVD) or vapor axial deposition (VAD).
Nevertheless, the MCVD process is often preferred since it affords greater flexibility in the choice of dopants and in the realization of selected refractive-index profiles; thus, for example, this technique enables realization of a jacket of lower refractive index, termed "depressed cladding", as well as of W-type and triangular profiles designed to shift the minimum dispersion into the spectral zone in which the silica has the lowest optical attenuation.
Various methods have already been proposed for reducing the paraxial refractive-index dip in the preform. Some of these methods involve a chemical attack upon the deposited silica inside the tube before or shortly after the start of the collapse. This expedient is only partly successful and encumbers the production by an additional process step. It has also been suggested to counteract the dip by excessively doping the innermost layers of the core or by letting the preform collapse into a flow of pure dopant. These operations, too, yield only partial results and are not readily reproducible. Furthermore, the flow of dopant through the preform cannot be maintained after one end is blocked by the beginning collapse so that the additional dopant blown into the core matrix necessarily concentrates on its remaining inner surface from which it again partly evaporates as the temperature increases.